Organic Light-Emitting Diodes (OLEDs) are very important in applications such as a light-emitting substrate and a display substrate. An OLED generally comprises a plurality of thin organic layers (polymer or a small molecular compound) sandwiched between a pair of electrodes. For example, at least one of these electrodes is transparent for the light emitted.
However, in a common OLED, since light emitted is mostly reflected or absorbed by the interior or an interface of the OLED rather than emitted outwardly, about three quarters of the light is trapped in the interior or at the interface of the OLED, and the external quantum efficiency thereof is less than 25%.